In-Line And Transverse Forces On Cylinders Near A Wall In Oscillatory Flow At High Reynolds Numbers

Abstract

ABSTRACT The in-line and transverse forces acting on circular cylinders placed near a plane boundary in a sinusoidally oscillating fluid in a U-shaped vertical water tunnel have been measured. The drag and inertia coefficients for the in-line force have been determined through the use of the Morison's equation. The transverse force has been expressed in terms of a single lift coefficient. The results have shown that the effect of wall-proximity is to increase the force coefficients for-relative gaps of e/D less than about 0.5. In this range of e/D values (0<e/D?0.5) the non-linear interaction between the shear layers emanating from the top and bottom of the cylinder is reduced and the frequency of oscillations in the shear layers is a synchronised. For e/D larger than about 0.5, the regular vortex shedding resumes, more or less unimpeded by the presence of the wall, and the lift, drag and inertia coefficients nearly assume their free-cylinder values. INTRODUCTION There are numerous problems associated with the design and installation of pipelines in deepwater offshore. The results such as the ones presented herein touch upon only one facet of the submerged pipeline design and do not deal with the consequences of pipeline forces due to temperature, pressure, seabed friction, buckling, scour, hydroelastic oscillations, etc. Suffice it to say that the reliable design information is most likely to come from a combination of laboratory studies, field experience, and careful documentation of the causes of failures. The present investigation is fn extension of that previously reported by this writer1 and was undertaken for the purpose of determining the in-line and transverse forces un cylinders near a wall at relatively higher Reynolds numbers. It is a well-known fact that for very small values of the period parameter UmT/D, the flow about the cylinder does not separate, the wake-dependent drag force is very small or negligible and the in-line as well as the transverse forces are essentially from the wave temporal acceleration. Under such circumstances, the appropriate coefficients may be easily evaluated from the potential theory. When the cylinder touches the boundary, a net force exists away from the wall. However, if even a very small gap exists between the cylinder and the wall, then a large net force exists toward the wall. The inertial forces alone do not usually give rise to the maximum load situation for small structures, such as pipelines located on or near ocean bottom, and the separation effects become extremely important not only in the determination of the magnitude but also the direction of the forces. It is important to recognize that the separation effects are not free from the effects of the boundary layer which forms on the bottom plane.